The present invention relates to a network storage system and a handover method between a plurality of network storage devices.
A network storage device is a storage device that is connected to a communication network such as the Internet or a LAN that is shared by a plurality of computers. NAS (Network Attached Storage) or similar is a known example of a network storage device. In organizations such as enterprises, municipalities, or schools, one or a plurality of network storage devices are introduced to construct a network storage system in order to manage enormous amounts of data.
Because of the volume of data to be managed by such organizations increases on a daily basis, the administrator of a network storage system is, before long, confronted by a shortage of network storage capacity. Alternatively, in cases where the number of clients using a network storage device has increased and so forth, the administrator examines improvements to the response of the network storage device.
In order to accommodate the shortfall in network storage capacity and the drop in response, and so forth, the existing network storage device is replaced with a new network storage device or the existing network storage device is used in combination with a newly added network storage device (Japanese Patent Application Laid Open No. 2004-46661).
With the conventional technology appearing in Japanese Patent Application Laid Open No. 2004-46661, the hierarchical directory structure of the existing network storage device, that is, the file tree structure thereof, is first copied to the newly installed network storage device and then files are transferred from the existing network storage device to the newly installed network storage device in accordance with an access request from the client.
When the client attempts initial access to a desired file after a reboot or the like, the client instructs the network-storage file system of the name of the object file and the directory name to which the file belongs. The file system then sequentially follows the file tree, which is a hierarchical directory structure, on the basis of information designated sequentially by the client and thus detects the requested file. For example, in cases where the client attempts access to file C that exists in a subdirectory B below directory A, the file system follows the file tree sequentially in the order of directory A, then subdirectory B, and then file C, and thus finds the object file C.
In order to allow the file system to subsequently access the file easily, file identification information (a file handle, or the like, for example) that is uniquely identified within the file system is issued and sent to the client. In the process in which the file system sequentially follows the file tree, file identification information for specifying each directory and file is communicated to the client.
The client then holds the file identification information issued by the file system in memory. As mentioned earlier, the file identification information is information for directly designating a file in the file system. Therefore, when the file is next accessed, the client requests the object file by directly designating the file identification information with respect to the network storage device. The file system directly specifies the requested file on the basis of the file identification information and thus provides the client with access to the file. Therefore, when valid file identification information has been inputted, the client need not search for the file by following the file tree sequentially.
Likewise, the newly installed network storage device is able to specify directly the file on the file system on the basis of file identification information. However, the file identification information is merely information uniquely identifying a file on the file system. It is not possible to specify directly the position of the file in the file tree by means of file identification information alone. Further, the file identification information basically functions effectively on the file system that issued the file identification information and cannot be used on a different file system.
Therefore, even when file access has been requested by the client, the newly installed network storage device is unable to specify the requested file in cases where a file is designated by using file identification information issued by a file system other than the file system of the newly installed network storage device or in cases where the requested file does not exist on the newly installed network storage device.
That is, the file system of the newly installed network storage device is unable to specify where the requested file is located in the existing network storage device on the basis of the file identification information issued by the file system of the existing network storage device alone and is therefore unable to access the substance of the requested file.
Therefore, in the technology appearing in Japanese Patent Application Laid Open No. 2004-46661, access by the newly installed network storage device to a file that has not already been transferred from the existing network storage device to the newly installed network storage device is difficult.
Hence, in the case of the technology appearing in Japanese Patent Application Laid Open No. 2004-46661, it is not enough to simply copy the file tree structure of the existing network storage device to the newly installed network storage device beforehand. It is also necessary to transfer the data of the whole file tree (or a predetermined range thereof), from the pre-specified upper directory to the lower directory, before starting file sharing services from the newly installed network storage device. In other words, file sharing services can be provided only within the range in which the data is transferred to the newly installed network storage device.
Therefore, in the case of the technology appearing in Japanese Patent Application Laid Open No. 2004-46661, file sharing services cannot be restarted during the interval up until the data transfer of the whole file tree from the existing network storage device to the newly installed network storage device is complete. As a result, replacement of the network storage device takes a long time. Further, with the technology appearing in Japanese Patent Application Laid Open No. 2004-46661, the data must have already been transferred before services are restarted, meaning that data cannot be transferred while file sharing services are being provided to the client.
Furthermore, the technology appearing in Japanese Patent Application Laid Open No. 2004-46661 also proposes virtually unifying the new network storage device by means of a virtual unification function with which the newly installed network storage device is provided, for example, and allocating an access request to each of the new and old network storage devices according to the priority of the requested file. However, although this virtual unification is significant from the perspective of the practical application of assets, the existing network storage device cannot be completely replaced with a newly installed network storage device. Therefore, an old file group that has been stored in the existing network storage device must be supplied by the existing network storage device whose access performance is relatively poor, and hence the response of the whole system drops. Further, the old-type network storage device with inferior performance must be maintained indefinitely, which increases the maintenance and running costs of the system.